How to use the rebar calculator
Four quick steps turn your slab dimensions into a two-way rebar mat — bars each way, grid ties, lap splices, steel weight, and the stock bars to buy — with a downloadable takeoff.
Set the rebar spec
Pick a bar size (#3–#6), the on-center spacing, and the clear cover. The grid runs both directions at your spacing.
Add your slabs
Enter each slab as length × width, with a quantity for identical pours. Add L-shapes as separate rectangles.
Set splices & waste
Choose your stock bar length and lap length, then dial in a waste factor for offcuts and bends.
Read results & save a PDF
Get bars each way, tie points, lineal feet, steel weight, and total bars — then download a two-page takeoff.
How a rebar takeoff is calculated
A slab mat is bars in two directions on a grid. Count each way from the clear span, total the lineal feet, add laps, then convert to weight and stock bars. Here is the exact math.
Worked example — a 20 ft × 20 ft slab, #4 @ 12″ o.c., 3″ cover, 20 ft stock, 10% waste
Clear span: 20 − 2×0.25 = 19.5 ft each way
Bars each way: floor(19.5 / 1) + 1 = 20 → 40 bars, 400 tie points
Lineal feet: 40 × 19.5 = 780 ft (runs under 20 ft, no laps)
Weight: 780 × 0.668 = 521 lb · Bars: 780 × 1.10 / 20 = 43
Order 43 × 20 ft #4 bars — about 780 lineal feet and 521 lb of steel.
Rebar estimating charts
Handy lookups for bar sizes and weights, bars per span at common spacings, steel weight, and typical slab reinforcement. Sizing is structural — treat these as starting points, not a design.
| Bar | Diameter | Weight | Typical use |
|---|---|---|---|
| #3 | 3/8 in | 0.376 lb/ft | Light slabs, sheds, ties |
| #4 | 1/2 in | 0.668 lb/ft | Residential slabs & patios |
| #5 | 5/8 in | 1.043 lb/ft | Driveways, garages, footings |
| #6 | 3/4 in | 1.502 lb/ft | Heavy footings & walls |
The bar number is the diameter in eighths of an inch. A 40× diameter lap is about 15″ for #3, 20″ for #4, 25″ for #5, and 30″ for #6.
| Clear span | @ 12″ o.c. | @ 18″ o.c. | @ 24″ o.c. |
|---|---|---|---|
| 10 ft | 11 bars | 7 bars | 6 bars |
| 15 ft | 16 bars | 11 bars | 8 bars |
| 20 ft | 21 bars | 14 bars | 11 bars |
| 30 ft | 31 bars | 21 bars | 16 bars |
| 40 ft | 41 bars | 27 bars | 21 bars |
Counts are one direction along the clear span (floor(span ÷ spacing) + 1). A two-way mat runs both directions, so total bars is the sum of both.
| Bar | Per foot | Per 20 ft bar | Feet per ton |
|---|---|---|---|
| #3 | 0.376 lb | 7.5 lb | ~5,320 ft |
| #4 | 0.668 lb | 13.4 lb | ~2,990 ft |
| #5 | 1.043 lb | 20.9 lb | ~1,920 ft |
| #6 | 1.502 lb | 30.0 lb | ~1,330 ft |
Suppliers often price rebar by weight, so lineal feet × pounds-per-foot is the figure to quote.
| Application | Common bar & spacing |
|---|---|
| Light slab / shed floor | #3 @ 18″ or wire mesh |
| Residential slab / patio | #4 @ 12–16″ o.c. |
| Driveway / garage slab | #4–#5 @ 12″ o.c. |
| Footing | 2–4 continuous #4–#5 bars |
| Clear cover (cast on earth) | ~3 in |
Bar size, spacing, cover, and lap length are structural decisions. These are typical starting points only — follow your engineer and local code.
Everything the calculator works out
One slab spec gives you every number needed to order and price the steel — from bars each way to total weight and stock bars.
Key figures every rebar takeoff depends on
Built for any slab pour
From a backyard patio to a run of driveways, the same calculator scales to the mat.
Pouring a patio or shed floor and wants the exact bars, tie wire, and total steel to buy.
- Enter one slab’s length × width
- Pick #4 @ 12″ to start
- Get bars each way and weight
Pricing driveway and garage slabs and needs bar counts, laps, and weight fast.
- Batch identical pours with Qty
- Set stock length & lap factor
- Hand the PDF takeoff to the yard
Building a rebar allowance into a bid, where suppliers quote by the pound.
- Total lineal feet across slabs
- Read steel weight in lb & ton
- Compare bar sizes and spacings
7 tips for a rebar mat that holds
Rebar only works if it’s the right size, in the right place, and held there through the pour. These habits keep your grid true and your order right.
Rebar calculator FAQ
The quantity, size, spacing, and splice questions people ask most before ordering steel.
For a two-way mat, count the bars in each direction, total the lineal feet, then add laps and waste.
Bars one way = floor(clear span ÷ spacing) + 1, where clear span = dimension − 2 × cover. A 20 ft × 20 ft slab with 3″ cover has a 19.5 ft clear span, so at 12″ o.c. that’s 20 bars each way — 40 bars, about 780 lineal feet.
Multiply lineal feet by the bar’s weight per foot for tonnage, and divide the waste-adjusted footage by your stock length for bars to buy.
The US bar number is the diameter in eighths of an inch, and the size is a structural choice — but common practice gives a starting point:
• #3 (3/8″): light slabs, sheds, or ties
• #4 (1/2″): most residential slabs and patios
• #5 (5/8″): driveways, garages, footings
• #6 (3/4″): heavy footings and walls
#4 at 12–16″ o.c. is typical for a residential slab. Always confirm size and spacing with your engineer and local code.
Spacing is measured on-center — center of one bar to center of the next — and run the same both directions to form a grid.
12″ o.c. is a common residential slab grid. Lighter mats go to 18″, and heavy reinforcement tightens to 6–10″. Closer spacing means more steel — halving the spacing roughly doubles the bar count.
The right spacing depends on load, thickness, and soil, so follow your structural design.
A lap splice is the overlap where two bars join on a run longer than one stock bar. A common rule of thumb is 40 × the bar diameter for a tension lap.
For #4 bar (1/2″) that’s about 20″, or 1.7 ft, added at each splice. Splices per bar = ceil(run ÷ stock length) − 1.
Stagger splices so they don’t all line up, and tie each lap. Lap length and class are structural — confirm them with your engineer.
Rebar weight is set by bar size, in pounds per linear foot:
• #3: 0.376 lb/ft • #4: 0.668 lb/ft
• #5: 1.043 lb/ft • #6: 1.502 lb/ft
Multiply total placed lineal feet (including laps) by the weight per foot. For example, 780 ft of #4 ≈ 521 lb, about a quarter ton. Suppliers often price by weight, so lineal feet × lb/ft is the figure to quote.
Clear cover is the concrete between the bar and the nearest surface — it protects the steel from corrosion and sets how far the grid is held in from each edge.
About 3″ is typical for concrete cast against earth, 1.5–2″ for formed surfaces, and less for interior faces. More cover means a slightly smaller grid and shorter bars, since clear span = dimension − 2 × cover.
Cover requirements come from your code and exposure, so verify them for your slab.
Divide your total lineal feet — including laps and a waste factor — by the stock bar length, then round up.
Rebar commonly comes in 20 ft lengths, with 10, 30, and 40 ft also available. Example: 780 ft plus 10% waste is 858 ft; at 20 ft stock that’s 43 bars.
Longer stock means fewer splices on long runs. Add 10% or more for offcuts, bends, and miscounts — steel is cheaper than a second delivery.
Both control cracking, and the right choice depends on the slab and the load.
Welded wire mesh is quick to place and suits light, non-structural slabs like small patios and shed floors. Rebar in a tied grid gives more control over placement and strength, and is the norm for driveways, garage slabs, and anything carrying real load.
Whichever you use, it has to sit up in the concrete on chairs, not on the ground. Follow your engineer or local code for what the slab requires.
Structural & Estimating Notice: This calculator estimates a two-way rebar mat — bar counts, lineal feet, weight, and stock bars — from your input and standard bar data. Bar size, spacing, clear cover, lap length and class, and whether a slab needs reinforcement at all are structural requirements that must follow your engineer’s design and local building code. It is a material takeoff, not an engineering design. Always verify the reinforcement schedule and quantities with your engineer or building department before ordering steel. For planning purposes only.